In previous estimations of the relative syntheses of the sulfur and 4-carbon moieties of methionine (made with separate cultures of Lemna), effects of biological variation could not be totally excluded. This problem has now been overcome by determination of these syntheses in the same culture of Lemna. It was confirmed that methionine inhibits de novo synthesis of its sulfur moiety more than its 4-carbon moiety, thereby showing that methionine is synthesized by a pathway (termed "preferential methionine sulfur recycling") in addition to the established one through transulfuration. The spermidine content of Lemna is about 1/2 of the amount of methionine metabolized through preferential methionine sulfur recycling, and it is suggested that a phsyiological role for this cycle in plants is to conserve methionine sulfur and prevent the accumulation of methiylthioadenosine formed during polyamine synthesis. It was further shown that transmethylation is the dominant pathway for methionine metabolism, accounting for 4 to 5 times the amount of methionine accumulating in protein. These are the first quantitative estimates in a higher plant of the relative extent of methionine metabolism through major pathways.